Hi Maurits,
if you have a google search you can see a lot of reasons that can generate this error.
You can check:
1.- You have enough space in your server.
2.- Your wp-content folder is writable (go to dashboard Appearance->editor and check if you can modify any file)
3.- Security’s plugins can filter the download process. You can try to disable these plugins.
4.- You might want to enable debugging to see if there are any error messages showing up in the debug.log when you try to install the integration.

If you go to point 4, please send us the debug.log file to support at itthinx dot com and dashboard admin access, then we’ll have a look (please indicate this topic in the email).
Thanks,
Antonio B.